Osteoporosis affects 15-20 million women in the United States. Prevention of this disease would be more beneficial and economical than treating the disease once bone loss and fractures have occurred. Dietary calcium has been implicated in the regulation of bone turnover. Inadequate calcium intake has been postulated to be important in the development of osteoporosis in women. Studies have reported that increased calcium intake in premenopausal women is associated with increased bone mass and results in greater bone mass as these women enter menopause. In a randomized, prospective 3-year study, diet modification with dairy products has been shown to retard vertebral bone loss in 30- to 42-year- old premenopausal women-compared to age-matched controls. This resulted in a statistically significant 2.9% difference in vertebral bone mass in the supplemented group after 2.5 and 3 years. Increased dietary calcium intake was reflected in increased urinary calcium excretion. Because the non-supplemented women lost bone at a rate of 2.9%/3 yr, prevention of bone loss by calcium supplementation during the premenopausal years could result in greater bone mass as these women enter menopause decreasing their risk of future fracture. It is the hypothesis of this proposal that calcium supplementation in women who have attained their peak bone mass will prevent the subsequent bone loss that occurs prior to menopause. This proposal will be a randomized, double-blind, placebo controlled intervention trial in 150 women to: (1) Determine the rates of vertebral, femoral (neck, trochanter, Ward's triangle), and forearm bone density changes in premenopausal Caucasian and Hispanic women, age 35-43; (2) Assess the effect of calcium supplementation (900 mg Calcium/d) on bone loss in these women; (3) Examine changes in markers of osteoblast (osteocalcin) and osteoclast (TRAP, type I collagen cross-linked N-telopeptides) activity in these women.